The future of forced draught burners

Published: 02 June, 2016

Steve O’Neill, Technical specifications engineer at EOGB Energy Products Ltd, focuses on energy efficiency in non-domestic buildings and how forced draught burners still play a role in reducing emissions.

Besides the automotive, steel and power generating industries, the provision of heating and hot water in commercial and non-domestic buildings is possibly the largest energy consumer across Europe.

Heating and hot water generation can be accountable for approximately 50% of the energy consumption within non-domestic buildings and is therefore accountable for its resultant greenhouse gas emissions into the atmosphere.

Energy savings

In October 2009 the EU introduced directive 2009/125/EC, with the aim of achieving a 20% reduction in carbon emissions by the year 2020.

The ErPD (Energy Related Products Directive), which came into force in September 2015, requires manufacturers of energy-related products to meet minimum energy performance criteria and nitrogen oxide (NOx) emissions. These standards apply to equipment designed for the heating of hot water and the heating of commercial/non-domestic buildings with a heat output of up to and including 400kW.

EU prognosticators forecast that by introducing such measures to drive up efficiencies, the 20% target will be achieved. As a result this will be conducive in reducing harmful emissions being emitted into the atmosphere.

This directive is going to have a significant and deleterious impact on some HVAC and process manufacturers as it can mean completely rethinking the direction in which a company must go to retain its position in the future market place. This is particularly true for manufacturers of forced draught burners.

Burners

A burner is a device that enables a chemical reaction between a fuel and an oxidizing agent to take place in a controlled environment. The burner acts as a means of stabilising the flame and controlling the intermixing of the combustion air and fuel.

Early burners were used to burn wood or solid fuel and it was discovered that altering the air to the burner could allow greater control of the flame and its temperature. It is this concept that is manipulated to enable us to control a flame and design equipment to suit a prescribed application.

Furthermore, this is known to be one of the fundamental techniques in improving the efficiency of the combustion process and one which consequently enables us to have an impact on the quantity and type of emissions being released into the atmosphere.

In order to fully consider the future of a specific type of burner it is necessary to consider what a burner is designed to do. A burner’s controlled heat release allows heat distribution, which can be applied to different functions varying from domestic cooking to industrial processes.

Fully pre-mixed flame

With this type of burner, all the combustion air is provided as primary air and they are commonly known as pre-mix burners.

Air under pressure (about 70mbar) entrains low-pressure gas. As the air pressure is increased it becomes easier to achieve near stoichiometric combustion due to the air for combustion being delivered direct to the burner producing a short, lean and intense flame.

As all of the air for combustion is pre-mixed with the fuel prior to combustion taking place, this increases the burning velocity and consequently produces a smaller, more compact high-temperature flame than that of an atmospheric burner.

The increased burning velocity reduces the time for the combustion process to complete, allowing for more fresh mixture to be introduced to the burner. This increased burning velocity allows for a greater energy release per unit time.

The smaller flame produced is due to the fact that it does not have an outer mantle, which can be advantageous as the combustion chamber can be made physically smaller reducing the risk of flame impingement.

This type of burner allows for simple self-proportioning by mechanical means of adjusting airflow (damper) or by utilising air/gas ratio valves (sometimes both).

Forced draught burners

With forced draught burners the fuel and air do not mix until they enter the burner head. This technique is applied for both liquid and gaseous fuels to great effect.

Both oxidizer and fuel are generally delivered to the burner head at pressure, which provides turbulent mixing of the fuel and oxidizer. The benefit of turbulent mixing is enhanced flame stability and it also increases the surface area of the flame front.

The increased surface area allows for a greater quantity of fuel to be burned per unit time than that of a laminar flame usually promoted in atmospheric and fully pre-mixed flames.

This type of burner can be set up to burn with either excess air or fuel depending on application. The rapid mixing process results in maximum heat release rates and short flames.

Nozzle mixing burners are used on all types of high temperature operations such as metal melting furnaces, heat treatment furnaces and in general where rapid heating is required.

The future of forced draught burners

Although the ErPD appears to prefer the fully condensing pre-mix burner for the provision of heat and hot water generation for commercial and non-domestic buildings, a need still exists for forced draught burners.

Compared to pre-mix, forced draught burners are more diverse as they are compatible with a wider range of applications. They will also always be a required for a number of industrial applications. With correct matching, forced draught burners can achieve the same efficiency as pre-mix burners, and even more with the addition of technology such as O2 and CO trim.

A lot of the efficiency from forced draught burners is lost in maintaining higher stack temperatures to prevent condensation forming inside a boiler that is not designed to dispose of it. In this situation it is normally possible for the burner to offer a greater turndown ratio than that which the boiler is designed to cope with.

Overall, both types of burner can offer optimum efficiency towards the application that they are designed for, irrespective of heat output. A more significant consideration is to ensure the appliance and its ancillary equipment are designed, specified, installed and commissioned correctly to suit the application.

The need for a wide variety of different burner types still exists and it is the application that will ultimately dictate the type of burner that is required.

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